symfony2 form multiple select with arraycollection
For a choice of entities you should use the special choice field type 'entity' (see Symfony manual for entity field type). For an example see my answer to a similar question.If you get further errors you may also find this question on Role Interface and Manage Roles helpful.
for the fosuserbundle I do it like that:
$builder->add('roles', 'choice', array( 'multiple' => true, 'choices' => array( 'ROLE_USER' => 'User', 'ROLE_AUTHOR' => 'Autor', 'ROLE_MODERATOR' => 'Moderator', 'ROLE_ADMIN' => 'Admin' ), 'label' => 'Rollen', 'required' => true ));